15 to 20 miles is pretty darned good for something with no moving parts. It would have worked pretty well against the WW1 Gotha bombers moving at around 100 mph under 10,000 feet, but would never give enough warning against modern WW2 bombers or even the slower Stukas. And this 200-foot concrete wall - obviously you can't re-point or aim the thing to check different approaches, although the mechanism to swing it around would have been impressive!
Would slaughtering seagulls at the shore be the same as poisoning pigeons in the park?
And if there is one avian species I'm less worried about going extinct than seagulls, it's hard to name - to quote a friend of mine, "Kill all you want - we'll make more".
Thanks for the welcome, glad to be aboard!